This invention relates to the interfacing of a gaseous source of sample material to an analyzer that requires a vacuum to operate.
Some analytical instruments require a high vacuum for successful operation, for example mass spectrometers. At the same time it is sometimes necessary to admit a certain amount of a gaseous sample for analysis from a high pressure region, often at atmospheric pressure. Any such inlet material needs to be pumped away by the high vacuum pump in order to maintain the vacuum required by the analyser. It is a feature of most vacuum pumps that they pump at a roughly constant volume flow rate and that the higher the flow rate required the more expensive the pump. This implies that a given mass flow rate is more expensive to pump away if the pressure at which the pump is operating is lower. For example, a vacuum pump operating at 10.sup.-5 mbar would have to have 10 times the volume rate capacity of a pump operating at 10.sup.-4 mbar in order to achieve the same mass flow rate.
In principle sample gas could be admitted from the high pressure source to the vacuum system through a single very small aperture with a single high vacuum pump operating at the pressure of the analyser. Such a leak would however have to be very small indeed and therefore difficult to interface to the source of analytical material. For example, suppose that the available pump capacity is 400 liter/sec (say a turbo-molecular pump weighing 20 pounds and costing some .English Pound.4000), the analyser requires to be at 10.sup.-5 mbar to operate successfully and the inlet is an aperture from atmospheric pressure straight into the vacuum. The effective pumping speed at atmospheric pressure is approximately 400 liter/sec.div.10.sup.8, the pressure ratio, which equals 4 .mu.liter/sec. A thin aperture from atmosphere to vacuum allows a volume flow rate of 200.times. A m.sup.3 /sec where A is the cross sectional area of the aperture in square meters. This implies an aperture area of 20 .mu.m.sup.2, or an aperture diameter of .about.5 .mu.m. Difficulties would arise because of the tendency of the leak to block, particularly if there are condensable components in the analytical stream. There may also be other reasons why the sampling aperture may not be this small. For example, in the particular case of sampling from an inductively coupled plasma, the sampling aperture must be larger than the plasma boundary layer in order to sample the plasma effectively (see J. A. Olivares and R. S. Houk, Anal. Chem. 57 p2674, 1985) leading to an aperture typically 0.5 to 1 mm. Often the pressure is reduced from atmospheric to the spectrometer operating pressure in more than one stage, such a system usually being referred to as a differential pumping system. Between each stage there is a small aperture, 0.1 to 1 mm in diameter, which separates a higher pressure region from a lower pressure region and each stage has its own pump. Typically the first vacuum stage is pumped with a rotary pump to 1 to 10 mbar, the second stage is pumped with a diffusion pump or turbo molecular pump to 10.sup.-4 to 10.sup.-3 mbar and the third stage is pumped with a high vacuum pump to 10.sup.-6 to 10.sup.-5 mbar. This way the bulk of the sample admitted is pumped away at relatively high pressures thus keeping down the capacity of the pumps used. Of course the consequence is that only a very small portion of the sample admitted through the first aperture actually travels all the way into the analyser space.
U.S. Pat. No. 3801788 discloses a method and apparatus for mass marking in mass spectrometry which provides molecule clusters at regular mass intervals over a mass range. U.S. Pat. No. 5049739 discloses a plasma ion source mass spectrometer with resonance charge exchange reaction and ion energy analysing sections which separate fast neutral atoms and slow disturbing ions. EP-A-0532046 discloses a vacuum device for a mass spectrometer with atmospheric pressure ionisation.
It is the object of the present invention to increase the proportion of the sample available to the analyser whilst at the same reducing the capacity and hence cost of the pumping required.